1. Field of the Invention
This invention relates to a selective ionization source to be used with, for example, an ion mobility spectrometry, an ionization detector and a mass spectrometry and more particularly to an electrolytic ionization source using inorganic/organic salts which react with sample molecules to form product ions.
2. Description of the Prior Art
The technique of ion mobility spectrometry (IMS) was conceived in the early 1970's in order to analyze and detect organic vapors. A typical ion mobility spectrometer (IMS) detector cell consists of a reaction region for generating ions and a drift region for separation of ions. In the reaction region, reactant ions are formed using radioactive materials such as, for example, tritium, Ni.sup.63, Am.sup.241, etc. High energy radiation from these radioactive materials ionize a carrier gas which flows through the reaction region to form reactant ions. Coronas from a multipoint or wire array, electrons produced by photoemission and multiphotoionization have also been proposed or used as methods to produce ions in IMS. The ions formed through these processes are of both polarities and the imposed electric field determines the polarity of the ions analyzed. In the absence of an electric field imposed on the reaction region, the recombination of the positive and negative ions predominates and ion concentrations are reduced to essentially zero. In the presence of an electric field, ion concentrations are non-zero as the electric field successfully competes to extract the ions from the reaction region. The nature of the reactant ions depends on the composition of the carrier gas. The composition of the carrier gas is selected to generate reactant ions with sufficient gas phase reactivity to allow a variety of reactions to occur between the reactant ions and sample molecules which may be introduced into the carrier gas for detection. The types of reactions which are available for this purpose are shown by equations 1-3 for positive ions:
Proton transfer EQU RH.sup.+ +M.fwdarw.R+MH.sup.+ ( 1)
Charge Transfer EQU R++M.fwdarw.R+M.sup.+ ( 2)
Nucleophilic attachment EQU R.sup.+ +M.fwdarw.MR.sup.+ ( 3)
Equations 4-8 show the type of reactions for negative ions:
Resonance capture EQU e.sup.- (.about.0.5 ev)+M.fwdarw.M.sup.- ( 4)
Dissociative capture EQU e.sup.- (.about.0.5 ev)+M.fwdarw.(M-A).sup.. +A.sup.- ( 5)
Charge transfer EQU R.sup.- +M.fwdarw.R+M.sup.- ( 6)
Proton abstraction EQU R.sup.- +M.fwdarw.RH+(M-H).sup.- ( 7)
Electrophilic attachment EQU R.sup.- +M.fwdarw.RM.sup.- ( 8)
In equations 1-8, R is the reactant moiety and M is the neutral sample moiety. After the ion/molecule reactions, a mixture of reactant ions and product ions exists in the reaction region.
A shutter grid positioned between the reaction region and the drift region permits momentary introduction of the ion mixture generated in the reaction region into the drift region. This is accomplished by momentarily removing a blocking voltage normally applied to the shutter grid. Once in the drift region, the ion mixture drifts under the influence of an electric field to an ion collector, Faraday plate, in a time characteristic for each ion as measured from the shutter grid. The drift times for the ions and the peak amplitudes in ion current arriving at the collector provide a basis for the identification of the chemical species originally introduced into the reaction region.
The IMS technique as described above has quite a few limitations. Some of the limitations are:
1. The variety of ion/molecule reactions available for ionization using radioactive sources does not provide specificity in the presence of interferences for detection,
2. Attempts to increase specificity by using non-radioactive sources, for example multiphotoionization and photoemission, results in reduced sensitivity for detection,
3. Corona discharge sources have proven to be an unreliable source of ions due to electrode sputtering processes,
4. Complex algorithms are needed to establish identification of samples with any present technique for providing a source of ions,
5. Sample introduction may require the use of a semipermeable membrane to eliminate effects of ambient air which is described in U.S. Pat. No. 4,311,669 which issued on Jan. 19, 1982 and is assigned to the assignee herein.
6. Use and handling of the radioactive materials must comply with U.S. Government regulations.
These limitations coupled with sensor design trade-offs, for example, sensitivity, selectivity, response time, service life, size, weight, power, etc. generally result in a compromised detection system for engineered applications. In a publication entitled "Selective Responses Of A Flameless Thermionic Detector" by Paul L. Patterson appearing in Journal of Chromatography, 167 (1978) 381-937, a flameless thermionic detector is described which uses an electrically heated bead consisting of an alkali metal compound embedded in a ceramic matrix. FIG. 6 shows chromatograms of a detector test sample at two different hydrogen flow-rates. Without any hydrogen, no response was observed from azobenzene and malathion.
In U.S. Pat. No. 4,378,499, issued on Mar. 29, 1983 to G. E. Spangler, D. N. Campbell and S. Seeb and assigned to the assignee herein, an Ion Mobility Detector is described in which selectivity and sensitivity is enhanced. As shown in FIG. 5 of '499, a reactive coating is applied to the internal wall of the reaction region. The reactive coating which may be activated by heating or by radiation from an ultraviolet source is selected to provide chemical conversion of sample molecules to a more ionizable form.
In U.S. Pat. No. 3,835,328 which issued on Sept. 10, 1974 to Harris et al. and in U.S. Pat. No. 4,075,550 which issued on Feb. 21, 1978 to Castleman et al. an ionization detector was described which utilized a radioactive source to produce beta radiation to form ions from a sample gas.
In a publication entitled "Atmospheric Pressure Ionization Mass Spectrometry" by D. I. Carroll et al. appearing in Applied Spectroscopy Reviews, 17 (3), 337-406 (1981), a general review of atmospheric ionization in mass spectrometry was provided. The use of an alkali salt was not mentioned.
Similarly, it is also desirable to provide an ion mobility spectrometer having a reaction and drift region with a doped solid electrolyte in the reaction region to react with sample molecules to form product ions.
It is further desirable to generate ions in certain equipment, for example, an ion mobility spectrometer ionization or mass spectrometer without the need for radioactive materials or without the need for hydrogen gas.
It is further desirable to provide an electrolytic ionization source which may be doped electrolyte where the electrolyte, such as alkali salts, can be adjusted to undergo general or class specific reactions with organophosphorous, nitrogen, and other organic or inorganic compounds.